Plasma reactors employed in semiconductor integrated circuit fabrication can use an electrostatic chuck (ESC) for holding the wafer inside the reactor chamber. Process control is enhanced by regulating the temperature of the semiconductor wafer held on the ESC. For example, a typical plasma etch process for forming high aspect ratio openings in the surface of a silicon wafer is carried out by introducing a process gas such as a fluorocarbon or fluorohydrocarbon gas into the chamber and coupling RF power into the chamber. Plasma RF source power for controlling plasma ion density may be applied by coupling VHF power to a ceiling electrode. Plasma RF bias power for controlling the plasma sheath voltage may be applied by coupling HF power to the ESC. In order to control the wafer temperature, an electrical heating element is provided within an insulating layer of the ESC as an electrically resistive element underlying the wafer support surface of the ESC. The RF bias power may be applied to a chucking electrode inside the insulating layer of the ESC. Alternatively, the RF bias power may be applied to a conductive base of the ESC that underlies the insulating layer of the ESC. In either case, some of the applied RF bias power capacitively couples to the electrical heating element, thereby diverting RF bias power away from the plasma. In fact, depending upon the design of the heating element, it is easier to couple RF bias power to the heating element than to the plasma. The electric heater circuit therefore is a significant RF load on the ESC or cathode. It changes the chamber impedance significantly. The RF current that is thus diverted flows through the heater current supply to RF ground. This diversion hampers control of the plasma because the plasma sheath voltage and ion energy (for example) are rendered uncertain and dependent upon the amount of capacitive coupling to the heater element, which may vary randomly.
In an effort to solve this problem, RF filters can be placed between the heating element and the heating current source. Such filters are designed to present a high impedance at the frequency of the RF bias power generator (typically but not necessarily at 13.56 MHz) to block RF current flow, while presenting little or no impedance to the 60 Hz heater supply current. In order to provide sufficient impedance at the RF bias frequency, commercially available RF filters typically include a choke or inductive winding around a magnetically permeable core about 0.65 mm in diameter having a very high permeability (e.g., a permeability within the range of 3000-7000, where permeability is the ratio between the permeability constant of the magnetic core and the permeability constant of air). Such a high permeability produces high magnetic flux in the core as a function of the RF voltage. We have found that at RF bias power levels required in typical plasma etch processes, e.g., 150 Watts at 13.56 MHz, the peak-to-peak RF voltage at the ESC can be as high as 2 kV. The magnetic flux in the core is a function of the RF voltage (2 kV) and the core permeability (4000), and is therefore very high. At such a high RF voltage, the high frequency (13.56 MHz) oscillation in magnetic field in the core causes extreme heating of the high permeability magnetic core and, ultimately, destruction and failure of the filter. We have found this problem in all of the commercially available RF filters we attempted to use with the ESC heater circuit. A solution to this problem did not seem possible because without a high permeability choke, the RF impedance at 13.56 MHz would be insufficient to prevent leakage of RF bias power through the heater circuit. For example, employing an air core choke (permeability of 1.0) would require over forty or more windings in the choke to provide sufficient inductive reactance. The problem with such an approach is that such a high number of turns in the choke winding would lead to a high capacitive reactance in the choke that would allow RF leakage.
Another problem is that the heater current, which can be as high as 40 amps, tends to heat the choke winding, which contributes to the problem of overheating in the RF filter.